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The subsidiary, AMEX Assurance Company, seeks to void a travel insurance policy that Gary Giordano, of Gaithersburg, took out in Robyn Gardner's name before their Caribbean vacation last year.

The lawsuit, filed last week in federal court in Maryland, says the policy isn't legally enforceable because Giordano and Gardner aren't married or otherwise related, weren't business partners and didn't own property together.

It also accuses him of lying on insurance forms by identifying Gardner as his "partner" when the two actually had what the company calls a "casual and non-exclusive" relationship.

"You have to have an economic interest in the person" when taking out a policy, American Express spokeswoman Gail Wasserman said Monday.

The suit also says that Gardner remains a missing person, and that aside from Giordano's public statements suggesting she is dead, "there is no other direct evidence that she is no longer alive."

Giordano, 51, was arrested at the Aruba airport in August 2011, several days after reporting her missing.

He has said Gardner, a Frederick woman he met on an adult social networking site, was swept out to sea as the two were snorkeling off the southern tip of Aruba - a claim contested by her family.

Giordano was jailed until late November, when a judge said prosecutors didn't have enough evidence to continue holding him.

He has since returned home to Gaithersburg, in suburban Washington. The travel insurance policy caught the attention of Aruban investigators trying to build a case against Giordano.

The policy listed Giordano as her sole beneficiary; Giordano designated his mother as his sole beneficiary.

Two days after reporting her missing, Giordano began "to make inquiries about the accidental death and dismemberment portion of the policy on the life of Ms. Gardner," according to the lawsuit.

Giordano has said he routinely takes out insurance when he travels and that he inquired about the insurance on the advice of his former lawyer. His lawyer, Scott Blumenshine, said Giordano remains entitled to collect the insurance sum.

In May, Giordano sued AMEX Assurance Company in Illinois, where the company is incorporated, seeking to collect $3.5 million under the terms of the policy.

Blumenshine said travelers are entitled to file claims on insurance policies they take out, and added that the claims in Giordano's lawsuit that would allow him to collect on the policy remain true.

The company says in its lawsuit that Gardner "had no role in the decision to purchase insurance." Giordano took out the policy, made himself the beneficiary and sent in the forms, said Wasserman, the American Express spokeswoman.